Adding words to your vim dictionary is quite simple. Choose the file you want to save your words (e.g. ~/.vim/dict.add) and add set the spellfile option:

:set spellfile=~/.vim/dict.add

This will create the file and a second file named ~/.vim/dict.add.spl which contains all the words you choose in vim-spell format. To add a new word to the dictionary, just place the cursor on in and press _ zg_. To mark a word as wrong, use zw.

You can manually add words to the spell-file by adding them to the textfile. After you did that, run

:mkspell %

and the .spl-file will be updated. You need to set the spell-file everytime you start vim. If you want to automatically load the spell-file, then place it into your ~/.vimrc.